Impact or pressure-sensitive carbonless transfer papers ordinarily have their back surfaces coated with microscopic capsules containing a reactive, colorless liquid capable of producing a mark when reacted with a front surface component of a lower, receiver sheet. The receiver sheet, placed in contact with the back face of the top sheet, has its front surface coated with a material having a component reactive with the contents of the capsules from the top sheet, so that when the capsules are ruptured upon impact by a stylus or machine key, the initially colorless or substantially colorless contents of the ruptured capsules spill out to contact and react with the surface reactant on the front (top) of the receiver sheet. Thus, a mark is formed on the receiver sheet corresponding to the mark impressed by the stylus or machine key on the top sheet.
In the art, impact transfer papers are designated by the terms CB, CFB and CF, which stand respectively for "coated back", "coated front and back" and "coated front". The C sheet is usually the top sheet and the one on which the impact impression is directly made; the CFB sheets are the intermediate sheets, each of which also transmits the contents of ruptured capsules from its back surface to the front of the next succeeding sheet; and the CF sheet is the last sheet and is only coated on its front surface to have an image formed thereon. The CF sheet is not normally coated on its back surface as no further transfer is desired.
While it is customary to coat the capsules on the back surface and to coat the coreactant for the capsules' contents on the front surface of each sheet, this procedure could be reversed, if desired. With some prior art systems, coatings are not used at all and the coreactive ingredients are carried in the sheets themselves, or one may be carried in one of the sheets and the other may be carried as a surface coating. Further, the reactants may both comprise microencapsulated liquids. Patents illustrative of many of the various kinds of systems which may incorporate such co-reactive ingredients and which may be used in the production of manifolded transfer papers include, for example, U.S. Pat. Nos. 2,299,695 to Green, 2,712,507 to Green, 3,016,308 to Macauley, 3,429,827 to Ruus and 3,720,534 to Macauley et al.
The most common variety of carbonless impact transfer paper, and the type with which the present invention is particularly useful, is the type illustrated, for example, in Green (U.S. Pat. No. 2,712,507) and Macauley (U.S. Pat. No. 3,016,308) wherein microscopic capsules containing a liquid fill comprising a solution of an initially colorless chemically reactive color forming dye precursor are coated on the back surface of the sheet, and a dry coating of an acid treated clay, reactive with the dye precursor, and extruded and acid treated in accordance with the present invention, is coated on the front surface of a receiver sheet.
It is well known that many naturally occurring clays may be acid treated to enhance their ability to develop color in carbonless copy paper. One of the most common clays used in this field is bentonite clays, particularly the non-water swelling bentonite clays such as calcium and magnesium bentonites. These acid activated bentonite clays are used for developing a color in carbonless copy paper systems as described in the following U.S. Pat. Nos.: 4,381,120; 4,221,690; 4,118,247; 4,111,461; 4,071,646; 4,042,412; 4,040,648; 4,028,133; 3,993,500 and 3,619,238.
The general process of acid activation of clays, as used today in industry, includes grinding the clay for uniform activation and dispersion in water; slurrying the ground clay in water and then adding acid to the clay slurry in an attempt to achieve a substantially uniformly activated clay. Usually about 6 to 8 hours of acid digestion is necessary to properly acid activate the clay. The digested slurry then is washed substantially free of dissolved salts such as iron and aluminum sulfates or chlorides, depending upon the particular mineral acid used. The acid activated clay then is dewatered, such as by filtration, and the resulting filter cake is dried and subsequently ground to product specifications.